The manufacture of glass-ceramics or semicrystalline ceramics involves the controlled crystallization of a parent glass by means of a special heat-treating process. In general, the process used to form a glass-ceramic consists of adding a nucleating or crystallization-promoting agent to a glass-forming batch, melting the batch and simultaneously cooling and shaping the glass melt into a glass body, and thereafter heat treating the glass shape pursuant to a specified time-temperature schedule. The special heat treatment results in the parent glass being converted into a body composed of fine-grained crystals randomly oriented and substantially uniformly dispersed throughout the glass matrix. The crystals comprise a major portion of the mass of the glass-ceramic. Glass-ceramics have found a number of uses due to the hardness, strength, resistance to thermal shock, resistance to strain, high softening point, and generally pleasing appearance exhibited by the glass-ceramic bodies produced. In addition, a number of conventional fashioning methods such as blowing, casting, drawing, pressing, or spinning can be used to fashion the glass ceramic while still a glass into a desired shape.
An example of the heat treating of a parent glass to produce a glass-ceramic body is disclosed in U.S. Pat. No. 3,201,266 (MacDowell). According to the disclosure of this patent, the crystalline phase or phases which are precipitated out of the glass during the heat-treating step directly influence the physical properties and characteristics of the glass-ceramic body. Another example of the classical method of using time-temperature to obtain a glass ceramic is disclosed in U.S. Pat. No. 3,410,674 (Martin). This patent concerns a process for producing seals, especially with materials having low thermal coefficients of expansion. Pressure is used to push a molten glass-ceramic between or onto a bonding surface. It has also been disclosed in U.S. Pat. No. 3,862,829 (Brandmayr et al) that the electrical properties of ferroelectric devitrifiable glass-ceramics are improved by subjecting the ceramics to inert gas pressures during formation.
The use of pressure in the formation of glass articles has also been disclosed in prior art. For example, in U.S. Pat. No. 3,145,090 (Buckner et al), temperature and pressure are applied to a molten glass to flow the glass into the desired shape without the devitrification which would occur if higher temperatures were used. In an article entitled "Effects of Very High Pressure on Glass" by P. W. Bridgman and I. Simon appearing in the Journal of Applied Physics Vol. 24, No. 4, April 1953, pages 405-413, it is mentioned that the thermal expansion of a glass may conceivably be affected by pressure. The use of forming pressures to control bubble formation in fused silica and ceramic materials is also disclosed in U.S. Pat. No. 3,247,293 (Myerson) and U.S. Pat. No. 1,536,821 (Devers).
The use of a hot pressing step in the formation of silica to obtain dimensional stability and to prevent the formation of cristobalite has been disclosed in U.S. Pat. No. 3,116,137 (Vasilos et al). In order to obtain desired optical properties in a ceramic, hot pressing can also be used as disclosed in U.S. Pat. No. 3,589,880 (Clark). A method for making heat-resistant transparent optical elements is also disclosed in U.S. Pat. No. 3,768,990 (Sellers et al) in which the optical element is subjected to high pressures.
The matching of the thermal expansion value of a glass to a metal to provide a good seal is disclosed in U.S. Pat. No. 2,234,551 (Dalton et al). According to the disclosure of this patent, where the glass for a mercury vapor light has a thermal expansion value significantly different from the thermal expansion value of the lead-in metal wire, a graded glass is used immediately adjacent to the metal wire. Thus, where the glass touches the metal wire, the thermal expansion value of the glass equals that of the metal wire. The thermal expansion value then varies regularly and continuously to a substantially different value at a distance from the metal wire.